<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      margin: 0 auto;
      width: 300px;
    }
    ul {
      list-style: none;
      padding: 0;
    }
    .list {
      float: left;
      margin: 0 10px;
    }
    .list ul {
      display: none;
    }
    .list li{
      border:1px solid #333;
    }
  </style>
</head>
<body>
  <ul class="box">
    <li class="list">
      <button>按钮</button>
      <ul>
        <li>123123</li>
        <li>456465</li>
        <li>789789</li>
      </ul>
    </li>
    <li class="list">
      <button>按钮</button>
      <ul>
        <li>123123</li>
        <li>456465</li>
        <li>789789</li>
      </ul>
    </li>
    <li class="list">
      <button>按钮</button>
      <ul>
        <li>123123</li>
        <li>456465</li>
        <li>789789</li>
      </ul>
    </li>
  </ul>

  <script>
    var lists = document.querySelectorAll('.list')

    for(var i = 0; i < lists.length; i++) {
      lists[i].onmouseover = function() {
        this.children[1].style.display = 'block'
      }
      lists[i].onmouseout = function() {
        this.children[1].style.display = 'none'
      }
    }
  </script>
  
</body>
</html>